One type of offshore production system includes a turret that lies within a cavity of a vessel, within the vessel hull or an extension thereof, the turret being moored by centenary chains and being connected through largely vertical risers that extend down to wells at the sea floor. The upper ends of those risers that carry fluids such as oil or gas, are connected through a fluid swivel at the top of the turret, to the vessel to deliver liquid and/or gaseous hydrocarbons to storage tanks in the vessel. A bearing structure consisting of one or more bearings rotatably connects the turret to the vessel, to allow the turret to avoid rotation while the vessel weathervanes around the turret (changes direction with changing winds, waves, and currents).
In prior systems which used a small number of risers, the turret could be designed so the risers extended up through a few vertical tubes. However, where a large number of risers must be accommodated, that extend from a plurality of wells, it is difficult to design an appropriate turret. It is theoretically possible to use a turret of very large diameter to provide a work area of about one meter between the upper ends of the risers. However, such turret of very large diameter would be heavy and costly and take up an appreciable portion of the vessel hull which otherwise could accommodate oil, as well as possibly requiring a vessel with a wider hull. A very important practical problem is that it is not presently possible to obtain bearings of more than about eight meters diameter. This is because very large equipment is used to forge and machine continuous raceways for the bearings, and applicant does not know of any source in the world which can supply larger precision bearings.